InGaAs/AlGaAs quantum well intermixing induced by Si impurities under multi-variable conditions

LIU Cui-cui，LIN Nan，MA Xiao-yu ，ZHANG Yue-ming，LIU Su-ping

（1.National Innovation Center of Radiation Application,China Institute of Atomic Energy, Beijing 102413, China；2.National Engineering Research Center for Optoelectronics Devices,Institute of Semiconductors, CAS, Beijing 100083, China；3.College of Materials Science and Opto-Electronic Technology, University of Chinese Academy of Sciences, Beijing 101408, China；4.Hitachi High-tech Scientific Solutions (Beijing) Co., Ltd., Beijing 100012, China）

Abstract: Catastrophic Optical Mirror Damage (COMD) on the cavity surface is the key factor limiting the threshold output power of high-power quantum well semiconductor laser diodes.To improve the output power of the laser diode,the band gap width of the active material in the cavity surface of the semiconductor laser diode can be adjusted by the quantum well intermixing technology to form a non-absorbing window transparent to the output laser.Based on the primary epitaxial wafers of InGaAs/AlGaAs high power quantum well semiconductor laser diode,using the single crystal Si dielectric layer grown by Metal Oxide Chemical Vapor Deposition (MOCVD) as the diffusion source,the research on Si impurity induced quantum well intermixing was carried out by using the Rapid Thermal Annealing (RTA) process.The effects of growth characteristics of Si dielectric layer,the temperature and time of RTA on the intermixing process were investigated.The experimental results show that the epitaxial 50 nm Si dielectric layer at 650 °C combined with 875 °C/90 s RTA treatment can obtain about 57 nm wavelength blue shift while maintaining the photoluminescence spectrum shape and the primary epitaxial wafers.It is found that the diffusion of Si impurities into the waveguide layer on the primary epitaxial wafer is the key to the remarkable effect of quantum well intermixing by the energy spectrum measurement technique.

Key words: semiconductor lasers;quantum well intermixing;rapid thermal annealing;blue shift;photoluminescence spectra

1 Introduction

In 1966,shortly after the advent of semiconductors,COOPERet al.[1]discovered that increasing the output power of GaAs homojunction semiconductor lasers to a certain level would result in Catastrophic Optical Damage (COD) and failure.In 1977,CHINONEet al.[2]discovered that an Al-GaAs/GaAs double heterojunction semiconductor laser operated continuously for a certain period resulted in Catastrophic Optical Mirror Damage (COMD)on its cavity surface.Using Scanning Electron Microscope (SEM) observation,it was found that high power density light output and cavity surface oxidation were important factors leading to its COMD[3].

For InGaAs/AlGaAs high-power Quantum Well(QW) semiconductor lasers,COMD suppression should start from its induced mechanism[4].According to test results,methods such as reducing non-radiative recombination at the cavity surface,suppressing light absorption of the cavity surface material,lowering the carrier concentration at the cavity surface,and improving the heat dissipation capacity at the cavity surface[5]can significantly suppress COMD.The preparation of non-absorbing windows based on Quantum Well Intermixing (QWI) technology is a low-cost and effective method to suppress the light absorption of cavity materials[6-7].Commonly used QWI methods include Impurity Induced Disordering (IID),Impurity Free Vacancy Induced Disordering (IFVD),Laser Induced Disordering (LID),etc.[8-11].Among them,in IID technique,a large number of point defects are induced by introducing impurities,and in combination with thermal annealing and other methods,the impurities and point defects are activated to obtain diffusion kinetic energy,ultimately causing changes in the composition and structure of quantum wells.In the 1980 s,LAIDIG[12]first found that QWI phenomenon occurred in AlAs/GaAs superlattice structures with the introduction of Zn impurities and heat treatment,and the heat treatment temperature in this method was only 575 °C,far below the temperature required for impurity free induced disordering.Until 1985,KALISKI[13]found that the effect of Si impurity inducing AlGaAs/GaAs superlattice QWI was better than that of other impurities.In 1987,MEIet al.[14]used Secondary Ion Mass Spectroscopy(SIMS) to test and found that the diffusion coefficient of Al atoms in AlGaAs materials increased significantly with the diffusion of Si impurities.Comprehensive research results show that Si impurities can form defect pairs with larger diffusion coefficients with Al atoms,and Si impurities can also increase the density of point defects in the QW system,thus effectively promoting the QWI of the AlAs/GaAs superlattice structure[6,15].

This paper presents a Non-Absorbing Window(NAW) preparation scheme for InGaAs/AlGaAs high-power QW semiconductor lasers using the method of Si impurity induced QWI.This method is based on the principle that the Si impurity is used as an induction source,which can efficiently induce the atomic interdiffusion between the materials in the QW and the materials in the barrier of the In-GaAs/AlGaAs semiconductor QW laser,eventually broadening the band gap of the active region material and suppressing its absorption of the self-generated laser.The preparation of NAW using the Si IID method not only reduces the optical absorption at the cavity surface of the laser,but also serves as an N-type doping element to form a non-carrier injection region at the cavity surface of the device,thus reducing the non-radiative composite here.This design does not require expensive equipment or complex processing,and can effectively increase the COMD threshold triggering power of the laser without changing its characteristic parameters.

2 Simulation analysis of QWI

2.1 Chip development and performance analysis

The primary epitaxial wafers of the InGaAs/Al-GaAs QW laser used in this paper were grown by Metal Oxide Chemical Vapor Deposition (MOCVD),with a reaction chamber growth temperature of 550-700 °C and a pressure of 5 kPa[16].The substrate is n-GaAs with a (100) plane offset [111]A-crystal-orientation of 15°.The schematic diagram of the ridge laser structure formed based on this primary epitaxial wafer is shown in Figure 1(color online).

Fig.1 Epitaxial structure of InGaAs/AlGaAs QW laser diode圖1 InGaAs/AlGaAs 量子阱激光器的外延結(jié)構(gòu)

For In(1-x-y)GaxAlyAs quaternary compound semiconductor material,its band gap is shown in formula (1),so the increase of Al component will lead to the increase ofEg.Therefore,we determine whether QWI has occurred in the material by the central wavelength position.If a QWI occurs,it is proved that the Al component has entered the QWI material,and the band gap becomes wider,which is shown by the change of the luminescence wavelength toward the short wavelength,that is,the blue shift occurs.

Photoluminescence (PL) spectroscopy test is a commonly used method to obtain the central wavelength of lasers.The original PL test results of the primary epitaxial wafer of InGaAs/AlGaAs QW lasers in this paper are shown in Figure 2.According to the mapping scan results,the luminescence intensity is uniform,indicating that the composition of each layer of the epitaxial wafer is uniform.From the single-point PL signal peak,it can be seen that the peak center wavelength is 1 002.2 nm,and the Full Width at Half Maximum (FWHM) is about 23 nm.

Fig.2 The PL spectrum of InGaAs/AlGaAs QW primary epitaxial wafer圖2 InGaAs/AlGaAs 量子阱初級外延片的PL 譜測試結(jié)果

2.2 The effect of temperature on QWI

The existence of point defects in crystals leads to the breaking of the perfect arrangement rules of lattice atoms,changes the vibration frequency of atoms around the defects,increases entropy,and deteriorates the thermodynamic stability[4].By combining the diffusion coefficient equation of group III atomic point defects,it can be concluded that:

whereAis a function related to the vibration entropyS fand vacancy,Bis a function related to the vibration entropyS fand interstitial atoms,EIis the energy required to form a interstitial atom,f1andf2are constants,is the diffusion coefficient of Group III vacancies,is the diffusion coefficient of Group III interstitial atoms,KBis the Boltzmann constant,and its value is 1.38×10-23J/K.Under the thermal equilibrium state,approximation can be considered as:A f1=Bf2,and2EV=E1,and the relationship curve between the relative interdiffusion coefficient of group III atoms and temperature can be fitted qualitatively according to formula (2),as shown in Figure 3.It can be seen that the diffusion coefficient of point defects in the group III-V material system is exponentially positively correlated with temperature,indicating that increasing the temperature is very beneficial for promoting the diffusion of point defects and enhancing the effect of QWI.

Fig.3 The relationship between relative interdiffusion coefficient and temperature圖3 III 族原子相對互擴散系數(shù)與溫度的關(guān)系

2.3 The effect of stress on QWI

At the interface of two materials with high lattice mismatch,there will be a certain amount of stress,which will cause compressive or tensile stress on the surface of the material.The surface compressive stress will cause the GaAs lattice atoms to be squeezed,and some atoms,especially Ga atoms,will be squeezed out of the interface,leaving a certain number of vacancy defects on the GaAs surface[17].To study the interface deformation during annealing process,the COMSOL multi-physical field modeling software was used to simulate the stress-strain behavior of GaAs with Si dielectric layers after annealing.

It is assumed that the epitaxial wafers are annealed at 850 °C,and stress is released when the annealing temperature drops to 200 °C,and finally stable deformation occurs at room temperature.The relevant parameters used in the calculation are shown in Table 1.The substrate material of the primary epitaxial wafer is 450 μm n-GaAs,the total thickness of the epitaxial wafer is approximately 4.5 μm,and both contain a large proportion of Ga and As components.To avoid calculation errors caused by excessive relative tolerance,the simulated substrate and epitaxial wafer are both 25-μm GaAs,with a dielectric layer of 200-nm Si.The simulation results based on COMSOL and magnified by 100 times are shown in Figure 4 (color online).It can be seen that the surface of GaAs undergoes compression caused by compressive stress after annealing,indicating that the Si dielectric layer will provide compressive stress to the GaAs surface and induce more Ga vacancies in GaAs,which is conducive to the QWI process.

Tab.1 Young's modulus,Poisson’s ratio,density and coefficient of thermal expansion of related materials表1 相關(guān)材料的楊氏模量、泊松比、密度及熱膨脹系數(shù)

Fig.4 Deformation results of primary epitaxial wafer simulated by COMSOL after annealing圖4 退火后初級外延片形變的COMSOL 模擬結(jié)果

3 Experiment and result analysis

3.1 Research on the effect factors of QWI

3.1.1 Effect of cover layer

During the annealing process,covering with GaAs cover plates not only reduces surface contamination,but also provides a certain pressure for As concentration,which can inhibit the decomposition and volatilization of As on the surface of the epitaxial wafer to some extent.The surface morphology of the primary epitaxial wafer at 875 °C/90 s RTA is shown in Figure 5 (color online).Figures 5(a) and 5 (b) show the surface morphology of primary epitaxial wafers with and without GaAs cover plates,respectively.Similar to the predicted results,the surface of epitaxial wafers with GaAs covers is smoother,and there are fewer ablative holes generated during annealing,indicating that the GaAs covers have a certain protective effect on the surface of the Si dielectric layer.Therefore,subsequent RTAs were conducted in the environment with GaAs covers.

Fig.5 Surface morphology (a) with and (b) without epitaxial wafers after RTA圖5 （a)有、(b)無蓋片退火后外延片的表面形貌

3.1.2 The effect of temperature

The calculation results show that temperature has a significant effect on the diffusion coefficients of impurities and point defects.Therefore,the effect of temperature on QWI is investigated first.By using MOCVD,a 20-nm single crystal Si was grown on the surface of GaAs primary epitaxial wafers at the growth temperature of 800 °C.Then,a 90 s Rapid Thermal Annealing (RTA) was performed in the interval of 775 to 900 °C,and the PL results after annealing are shown in Figure 6 (color online).It can be seen that the effect of wavelength blue shift increases with the increase of heat treatment temperature.Compared to the original primary epitaxial wafers,a maximum wavelength blue shift of about 90 nm was obtained at 900 °C,but at this point,the FWHM was significantly widened and the waveform was severely deteriorated,indicating significant material damage.At 875 °C,the wavelength blue shift is about 57 nm,and the FWHM is well maintained.Therefore,it is believed that heat treatment at 875 °C can achieve a good QWI effect while also ensuring the lattice quality of the material.

Fig.6 Effect of RTA temperature on wavelength blue shift of primary epitaxial wafers圖6 RTA 溫度對初級外延片波長藍移的影響

3.1.3 The effect of heat treatment time

The effect of heat treatment time on QWI is further investigated.The annealing temperature is always 875 °C,and the annealing time is set to 60 s,90 s,and 120 s respectively.The PL results of the primary epitaxial wafers after annealing are shown in Figure 7 (color online).As the annealing time increases,the wavelength blue shift of the primary epitaxial wafer introducing Si impurities also gradually increases.However,when the annealing time reaches 120 s,the peak of the PL spectrum is already deformed.It indicates that after 90 s RTA treatment,a good blue shift effect can be achieved,and the peak intensity of the PL spectrum and the FWHM remain good.

Fig.7 Effect of RTA time on wavelength blue shift of primary epitaxial wafers圖7 RTA 時間對初級外延片波長藍移的影響

3.1.4 The effect of the properties of the dielectric layer

If the Si grown on the epitaxial wafer surface is too thick,the lattice mismatch and the difference in coefficient of thermal expansion will be amplified,which will trigger the stress release during thermal annealing.The ability of a thinner Si layer to suppress the decomposition and outward volatilization of Ga and As atoms in the GaAs ohmic contact layer will also be weakened,so it is necessary to consider the effect of Si characteristics.The Si dielectric layer grown by MOCVD equipment is single crystal,and its lattice quality and density are affected by the reaction source,growth temperature and other conditions,so the Si dielectric layer grown under different conditions will also affect the QWI effect.Therefore,three types of Si epitaxial layers were prepared: 20 nm high-temperature Si grown at 800 °C,20 nm low-temperature Si grown at 650 °C,and 50 nm low-temperature Si,set as # 1,# 2,and # 3,respectively,to investigate the optimal growth conditions for Si dielectric layers that induce best QWI effect.

Similarly,a single RTA treatment at 875 °C/90 s was applied to the group of the primary epitaxial wafers,and the PL spectra of the primary epitaxial wafers were tested after heat treatment,as shown in Figure 8 (color online).It can be seen that the difference of QWI effect caused by the three types of Si layers is relatively small.For Si layers with the same thickness,the effect of Si layer growth temperature on wavelength blue shift is relatively small,but the FWHM is narrower for the high-temperature Si layers.For Si layers with the same growth conditions,thicker Si layers cause more wavelength blue shifts,reaching about 57 nm,but their FWHM is also larger,indicating that the material quality is greatly affected.

Fig.8 Effect of different silicon layers on wavelength blue shift of primary epitaxial wafers圖8 Si 介質(zhì)層對初級外延片波長藍移的影響

3.2 Microscopic characterization of QWI effect

In order to accurately understand the diffusion depth of Si atoms,EDS was used to test the element distribution at different depths on the epitaxial wafer.The Si IID primary epitaxial wafers treated with 875 °C/90 s RTA were carried out Si layer removal treatment,and then corroded for 0 s,15 s,30 s,and 45 s using a special solution.The test results are shown in Figure 9 (color online).Experience shows that the corrosion rate of the corrosive solution is about 25-35 nm/s,so the surface of the etched epitaxial wafer corresponds to different depths.From the EDS results,it can be seen that the p-type doping element of the primary epitaxial wafer is C,so the element C content is ligher when the surface layer of GaAs is not corroded moreover,the element Si content is also higher,and The content of both in the same order of magnitude;with the corrosion time increases to 15 s,the element C content gradually decreases,and the element Si content decreases significantly;when the corrosion time reaches 30 s,i.e.,when the corrosion depth reaches approximately the upper limiting layer,the Si content has decreased to 22.2% of the original Si content in the surface layer;when the corrosion time further increases to 45 s,i.e.,when the corrosion depth reaches approximately the upper waveguide layer or near the QW region,the Si content basically decreases to 0.This result shows that the Si impurities can diffuse into the upper waveguide layer of the primary epitaxial wafer after 875 °C/90 s RTA treatment,and then produce an effective QWI induction effect.

Fig.9 Surface EDS results of element composition at different corrosion times of primary epitaxial wafers after 875 °C/90 s RTA.(a) Untreated sammple;(b) corrosion for 15 s;(c) corrosion for 30 s;(d) corrosion for 45 s圖9 EDS 測試875 °C/90 s RTA 處理后初級外延片不同腐蝕時長的元素組成。（a）未處理樣品；（b）腐蝕15 s；（c）腐蝕30 s；（d）腐蝕45 s

4 Conclusion

In order to comprehensively improve the performance index of InGaAs/AlGaAs semiconductor QW lasers,a feasible scheme for Si impurity induced QWI is investigated in this paper.The relationship between the effect of Si impurity-induced QWI and the nature of dielectric layer and heat treatment conditions was investigated by using the variable-controlling method with multiple sets of control conditions.The PL test results show that growing a 50 nm Si epitaxial dielectric layer at 650 °C

——中文對照版——

1 引言

半導(dǎo)體問世不久的1966 年，COOPER 等人[1]in combination with 875 °C/90 s RTA heat treatment results in a wavelength blue shift of about 57 nm.Combined with EDS test,it is found that Si impurity atoms can diffuse into the upper waveguide layer or QW of the InGaAs/AlGaAs semiconductor QW laser primary epitaxial layer after 875 °C/90 s RTA,resulting in a significant QWI effect.In the future,Si impurity induced QWI NAW can be prepared by combining epitaxial growth technology and RTA technology to suppress CODs and continuously improve the output power of InGaAs/Al-GaAs semiconductor QW lasers.發(fā)現(xiàn)，當GaAs 同質(zhì)結(jié)半導(dǎo)體激光器的輸出功率升高到一定值時便會產(chǎn)生光學(xué)災(zāi)變損傷(Catastrophic Optical Damage,COD)并失效。1977 年，CHINONE 等人[2]發(fā)現(xiàn)AlGaAs/GaAs 雙異質(zhì)結(jié)半導(dǎo)體激光器連續(xù)工作一定時間后，在其腔面處將產(chǎn)生了腔面光學(xué)災(zāi)變損傷(Catastrophic Optical Mirror Damage,COMD)。使用掃描電子顯微鏡(Scanning Electron Microscope,SEM)觀測發(fā)現(xiàn)，高功率密度光輸出及腔面氧化是導(dǎo)致其發(fā)生COMD 的重要因素[3]。

對于InGaAs/AlGaAs 高功率量子阱(Quantum Well,QW)半導(dǎo)體激光器，抑制COMD 應(yīng)從研究其誘發(fā)機理入手[4]。經(jīng)檢驗，通過減少腔面處的非輻射復(fù)合、抑制腔面材料的光吸收、降低腔面處載流子濃度、提高腔面處散熱能力等方法[5]，均可顯著抑制COMD。基于量子阱混雜(Quantum Well Intermixing,QWI)制備非吸收窗口是一種成本較低、效果顯著的抑制腔面材料光吸收的方法[6-7]。常用的量子阱混雜方法包括雜質(zhì)誘導(dǎo)量子阱混雜(Impurity Induced Disordering,IID)、無雜質(zhì)誘導(dǎo)量子阱混雜(Impurity Free Vacancy Induced Disordering,IFVD)、激光誘導(dǎo)量子阱混雜(Laser Induced Disordering,LID)等[8-11]。其中，IID 技術(shù)是通過引入雜質(zhì)誘生大量點缺陷，并結(jié)合熱退火等工藝使雜質(zhì)及點缺陷激活并獲得擴散的動能，最終造成量子阱組分及結(jié)構(gòu)的變化。上世紀80 年代，LAIDIG[12]最先發(fā)現(xiàn)引入Zn 雜質(zhì)并經(jīng)熱處理的AlAs/GaAs 超晶格結(jié)構(gòu)發(fā)生了量子阱混雜現(xiàn)象，且此方法中的熱處理溫度僅575 °C，遠低于無雜質(zhì)誘導(dǎo)混雜所需溫度。直到1985 年，KALISKI[13]發(fā)現(xiàn)Si 雜質(zhì)誘導(dǎo)AlGaAs/GaAs 超晶格量子阱混雜的效果比其他雜質(zhì)更好。1987 年，MEI 等人[14]利用二次離子質(zhì)譜(Secondary Ion Mass Spectrometry,SIMS)測試發(fā)現(xiàn)在AlGaAs 材料中Al 原子的擴散系數(shù)會隨著Si 雜質(zhì)的擴散顯著上升。綜合研究認為，Si 雜質(zhì)能與Al 原子形成擴散系數(shù)較大的缺陷對，且Si 雜質(zhì)也可增加量子阱體系中點缺陷的密度，進而有效促進AlAs/GaAs 超晶格結(jié)構(gòu)的量子阱混雜[6-15]。

本文利用Si 雜質(zhì)誘導(dǎo)量子阱混雜的方法為通過InGaAs/AlGaAs 高功率量子阱半導(dǎo)體激光器提供非吸收窗口(Non-Absorption Window,NAW)。主要原理是采用Si 雜質(zhì)作為誘導(dǎo)源，高效地誘導(dǎo)InGaAs/AlGaAs 半導(dǎo)體量子阱激光器的量子阱區(qū)材料與壘區(qū)材料發(fā)生原子互擴散，最終使有源區(qū)材料禁帶寬度變寬，抑制其對自身產(chǎn)生的激光的吸收。利用Si 雜質(zhì)誘導(dǎo)量子混雜方法制備非吸收窗口不僅可以減少激光器腔面處的光吸收，也可以作為N 型摻雜元素在器件腔面處形成非載流子注入?yún)^(qū)，減少此處的非輻射復(fù)合。這種設(shè)計不需高成本的設(shè)備或復(fù)雜的處理過程，在不改變激光器特征參數(shù)的同時，可有效提高其COMD 閾值觸發(fā)功率。

2 量子阱混雜的模擬分析

2.1 芯片研制及性能分析

本文所使用的InGaAs/AlGaAs 量子阱激光器初級外延片采用金屬氧化物化學(xué)氣相沉積(Metal Oxide Chemical Vapor Deposition，MOCVD)生長，反應(yīng)室生長溫度為550～700 °C,反應(yīng)室壓強為5 kPa[16]。襯底為(100)面偏[111]A 晶向15°的 n-GaAs，基于該初級外延片形成的脊型激光器結(jié)構(gòu)示意圖如圖1 所示。

對于In(1-x-y)GaxAlyAs 四元化合物半導(dǎo)體材料，其禁帶寬度如公式(1)所示，故Al 組分增多會導(dǎo)致Eg增大。因此，本研究通過中心波長位置判定材料是否發(fā)生了量子阱混雜。若發(fā)生了量子阱混雜，證明量子阱材料中有了Al 組分，禁帶寬度變寬，表現(xiàn)為發(fā)光波長朝短波長變化，即藍移。

光致發(fā)光(Photoluminescence,PL)光譜測試是獲得激光器中心波長的常用方法，本文中In-GaAs/AlGaAs 量子阱激光器初級外延片的原始PL 測試結(jié)果如圖2。由其映射掃描結(jié)果可知其發(fā)光強度均勻。說明外延片各層成分均勻。由單點PL 信號峰可知峰值中心波長為1 002.2 nm，半高全寬(Full Width at Half Maximum,FWHM)約為23 nm。

2.2 溫度對量子阱混雜的影響

晶體中點缺陷的存在導(dǎo)致晶格原子的完美排列規(guī)則被打破，缺陷周圍的原子振動頻率發(fā)生改變，熵值增大，熱力學(xué)穩(wěn)定性變差[4]。結(jié)合III 族原子點缺陷互擴散系數(shù)方程可得：

其中，A是與振動熵S f及空位相關(guān)的函數(shù)，B是與振動熵S f及填隙原子相關(guān)的函數(shù)，EI為形成一個填隙原子所需要的能量，f1、f2是常數(shù)，是III族空位的擴散系數(shù)，是III 族間隙原子的擴散系數(shù)，KB為玻爾茲曼常數(shù)，其值為 1.38×10-23J/K。熱平衡狀態(tài)下可考慮存在以下近似：A f1=Bf2，2EV=E1。根據(jù)公式(2)定性擬合出III 族原子相對互擴散系數(shù)與溫度的關(guān)系曲線，如圖3 所示?？梢?，III-V 族材料體系內(nèi)點缺陷的擴散系數(shù)與溫度呈指數(shù)型正相關(guān)。證明升高溫度非常有利于促進點缺陷的擴散，提升量子阱混雜的效果。

2.3 應(yīng)力對量子阱混雜的影響

在兩種晶格失配度較大的材料界面處會存在一定應(yīng)力，從而使材料表面存在壓應(yīng)力或張應(yīng)力。而表面壓應(yīng)力會使GaAs 晶格原子受到擠壓，部分原子，尤其是Ga 原子會被擠壓出界面而在GaAs 表面留下一定數(shù)量的空位缺陷[17]。為了研究退火過程中的界面形變，使用COMSOL 多物理場建模軟件模擬了帶有Si 介質(zhì)層的GaAs 退火后的應(yīng)力應(yīng)變情況。

假設(shè)外延片經(jīng)過850 °C 高溫退火，在退火溫度降到200 °C 時產(chǎn)生應(yīng)力釋放，并最終在室溫下產(chǎn)生穩(wěn)定形變，計算使用的相關(guān)參數(shù)見表1。初級外延片襯底材料為450 μm 的n-GaAs，外延片總厚度約為4.5 μm，且均含大比例Ga、As 組分。為避免相對容差過大產(chǎn)生的計算錯誤，選用襯底及外延片為25 μm GaAs，介質(zhì)層為200 nm Si 用于模擬分析?；贑OMSOL 并放大100 倍后的模擬結(jié)果如圖4（彩圖見期刊電子版）所示。可見，退火后GaAs 表面出現(xiàn)了由壓應(yīng)力帶來的縮緊現(xiàn)象，這表明Si 介質(zhì)層會為GaAs 表面提供壓應(yīng)力，誘導(dǎo)GaAs 內(nèi)產(chǎn)生更多Ga 空位，這有利于量子阱混雜過程的進行。

3 實驗及結(jié)果分析

3.1 量子阱混雜的影響因素研究

3.1.1 蓋片層的影響

在熱退火過程中，加蓋GaAs 蓋片不僅能減少表面沾污，還能提供一定濃度As 壓。這可在一定程度上抑制外延片表面As 的分解及揮發(fā)。經(jīng)875 °C/90 s RTA 退火后的初級外延片表面形貌見圖5（彩圖見期刊電子版）。圖5(a)、5(b)分別為有、無GaAs 蓋片的初級外延片表面形貌。與預(yù)測結(jié)果相同，有GaAs 蓋片的外延片表面更加光潔。這是因為退火產(chǎn)生的燒蝕孔較少。說明GaAs 蓋片對Si 介質(zhì)層表面起到了一定的保護作用，故后續(xù)RTA 均在有GaAs 蓋片環(huán)境下進行。

3.1.2 溫度的影響

由計算可知，溫度對雜質(zhì)及點缺陷的擴散系數(shù)影響極大，故首先研究溫度對量子阱混雜的影響。利用MOCVD 在初級外延片GaAs 表面生長20 nm 單晶Si，生長溫度為800 °C。然后，在775～900 °C 區(qū)間進行90 s 快速熱退火(Rapid Treatment Annealing，RTA)處理，退火后的PL 結(jié)果如圖6（彩圖見期刊電子版）所示?？梢姡ㄩL藍移的效果隨熱處理溫度升高而增大。對比原始初級外延片，在900 °C 時獲得約90 nm 的最大波長藍移量，但此時FWHM 顯著加寬，波形嚴重惡化，說明材料損傷較大。而在875 °C 時波長藍移量約為57 nm，且FWHM 保持較好。故認為875 °C熱處理溫度可在獲得良好量子阱混雜效果的同時保證材料的晶格質(zhì)量。

3.1.3 熱處理時間的影響

繼續(xù)研究熱處理時間對量子阱混雜的影響。退火溫度均為875 °C，退火時間分別設(shè)為60s、90s、120s，退火后初級外延片的PL 結(jié)果如圖7（彩圖見期刊電子版）所示?？梢姡S著退火時間延長，引入Si 雜質(zhì)的初級外延片波長藍移也逐漸增加，但退火時間達到120s 時PL 譜峰已經(jīng)變形。圖7 表明經(jīng)90s RTA 處理可獲得較好的藍移效果，PL 譜峰值強度、FWHM 均保持較好。

3.1.4 介質(zhì)層性質(zhì)的影響

如果外延片表面生長的Si 過厚，則晶格失配及熱膨脹系數(shù)的差別會被放大，會觸發(fā)熱退火過程中的應(yīng)力釋放。而較薄的Si 層抑制GaAs 歐姆接觸層中Ga、As 原子分解和向外揮發(fā)的能力也會減弱，因此需要考慮Si 特性的影響。MOCVD設(shè)備所生長的Si 介質(zhì)層為單晶材料，其晶格質(zhì)量和致密度受到反應(yīng)源、生長溫度等條件的影響，故不同條件下生長的Si 介質(zhì)層也會影響量子阱混雜效果。因此，制備了3 種Si 外延層：800 °C下生長的20 nm 高溫Si、650 °C 下生長的20 nm-Si 和50 nm 低溫Si，分別設(shè)為#1，#2，#3，用于尋找誘導(dǎo)量子阱混雜效果最佳的Si 介質(zhì)層生長條件。

同樣，對該組初級外延片進行875 °C/90 s 單次RTA 處理，并在熱處理后測試初級外延片的PL 譜，見圖8（彩圖見期刊電子版）。可見，3 種類型的Si 層所引起的量子阱混雜效果差別較小。結(jié)果表明對于厚度相同的Si 層，Si 層生長溫度對波長藍移量的影響較小，但帶有高溫Si 層的FWHM 更窄。對于生長條件相同的Si 層，較厚的Si 層所引起的波長藍移更多，達到了57 nm 左右，但此時其FWHM 也較大，說明材料質(zhì)量受影響較大。

3.2 量子阱混雜效果的微觀表征

為了更準確地了解Si 原子的擴散深度，使用EDS 測試了外延片不同深度處的元素分布。先對875 °C/90 s RTA 處理的Si IID 初級外延片進行去Si 層處理，再使用特制溶液分別腐蝕0 s、15 s、30 s、45 s，測試結(jié)果見圖9（彩圖見期刊電子版）。經(jīng)驗表明：腐蝕液的腐蝕速度約為25～35 nm/s，腐蝕后的外延片表面對應(yīng)不同深度的區(qū)域。由EDS 結(jié)果可見，該初級外延片的p 型摻雜元素為C，故表層GaAs 未被腐蝕時C 元素含量較高，而Si 元素的含量也較高，且二者的含量在同一量級；隨腐蝕時間增加至15 s，C 元素含量逐漸減少，Si 元素含量則大幅度下降；當腐蝕時間為30 s，約腐蝕到上限制層時，Si 含量已下降至表層的22.2%；當腐蝕時間為45 s 時，約腐蝕到上波導(dǎo)層或接近量子阱區(qū)，此時Si 含量基本下降為0。該結(jié)果證明了經(jīng)875 °C/90 s RTA 處理，Si 雜質(zhì)可以擴散到初級外延片的上波導(dǎo)層區(qū)，進而產(chǎn)生有效的量子阱混雜誘導(dǎo)效果。

4 結(jié)論

為了全方面提高InGaAs/AlGaAs 半導(dǎo)體量子阱激光器的性能指數(shù)，本文研究了Si 雜質(zhì)誘導(dǎo)量子阱混雜的可行方案。利用控制變量法，設(shè)置了多組對照條件，研究了Si 雜質(zhì)誘導(dǎo)量子阱混雜 效果與介質(zhì)層性質(zhì)、熱處理條件等因素的關(guān)系。PL 測試結(jié)果顯示，在650 °C 下生長50 nm Si 外 延介質(zhì)層結(jié)合875 °C/90 s RTA 熱處理，可獲得 約57 nm 的波長藍移量。結(jié)合EDS 測試發(fā)現(xiàn)，875 °C/90 s RTA 后Si 雜質(zhì)原子可擴散到In-GaAs/AlGaAs 半導(dǎo)體量子阱激光器初級外延片 的上波導(dǎo)層或量子阱，因而產(chǎn)生顯著的量子阱混 雜效果。未來，可結(jié)合外延生長技術(shù)、RTA 技術(shù) 制備Si 雜質(zhì)誘導(dǎo)量子阱混雜非吸收窗口，抑制光 學(xué)災(zāi)變發(fā)生，持續(xù)提升InGaAs/AlGaAs 半導(dǎo)體量 子阱激光器的輸出功率。