Electrooptical properties of GaNAs/GaAs multiple quantum well structures

The electrooptical properties of the GaNAs/GaAs multiple quantum well structures have been studied using the photoreflectance spectroscopy from 20 K to room temperature. Above the band gap energy of GaAs, Franz–Keldysh oscillations were observed. The period of the Franz–Keldysh oscillations decreased slightly with decreasing temperature, and indicated that the corresponding space charge distribution varied slowly with temperature. The modulated quantum well transition features were observed below the band gap energy of GaAs. A matrix transfer algorithm was used to calculate the quantum well subband energies numerically. The band gap energy and the electron effective mass of the GaNAs/GaAs system were adjusted to obtain the subband energies to best fit the observed quantum well transition energies.     

Photoreflectance spectroscopy for the study of GaAsSb/InP heterojunction bipolar transistors

This work is devoted to a photoreflectance (PR) study of single GaAsSb layers grown pseudomorphically on InP. Such antimonide alloys included in the base of heterojunction bipolar transistors (HBT) are expected to lead to high cut-off frequency transistors. However, many important parameters such as the nature of the InP/GaAsSb interface, the conduction band offset and the Fermi level pinning at GaAsSb surface are not well known in this system. In this study, they are determined from optical measurements performed between 8 and 300 K. Unusual evolution of the photovoltage as a function of temperature is reported in GaAsSb. Finally, a PR study of a complete HBT structure shows the potential of this technique to non-destructively characterize complex epitaxial layers, and further on to study fully processed devices under operation.     

Polarized photoreflectance and photoluminescence spectroscopy of InGaAs/GaAs quantum rods grown with As2 and As4 sources

We report photoreflectance (PR) and photoluminescence (PL) investigations of the electronic and polarization properties of different aspect ratio (height/diameter) InGaAs quantum rods (QRs) embedded in InGaAs quantum wells (QWs). These nanostructures were grown by molecular beam epitaxy using As₂or As₄sources. The impact of the As source on the spectral and polarization features of the QR- and QW-related interband transitions was investigated and explained in terms of the carrier confinement effects caused by variation of composition contrast between the QR material and the surrounding well. Polarized PR and PL measurements reveal that the polarization has a preferential direction along the [11¯0] crystal axis with a large optical anisotropy of about 60% in the (001) plane for high aspect ratio (4.1:1) InGaAs QRs. As a result, in PL spectra, the transverse magnetic mode dominated (11¯0)-cleaved surfaces (TM_[001]>TE_[110]), whereas the transverse electric mode prevailed for (110)-cleaved surfaces (TM[001]<TE[11¯0]). This strong optical anisotropy in the (001) plane is interpreted in terms of the hole wavefunction orientation along the [11¯0] direction for high aspect ratio QRs.     

分子束外延GaAs/Si应变层的光致发光和光反射谱研究

研究了分子束外延GaAs/Si光致发光谱的激发强度和温度依赖关系。确定出2个本征发光峰,分别对应于导带至m_J=±3/2和m_J=±1/2价带的复合。这种价带的移动和分裂归因于由GaAs和Si的热膨胀系数不同所引起的GaAs层双轴张应力。还观测到4个非本征发光峰,分别为导带至m_J=±1/2碳受主态发光、可能与缺陷有关的发光以及可能由Mn和Cu受主杂质引起的发光。室温下将GaAs/Si和GaAs/GaAs材料的光反射谱进行比较,前者明显向低能移动约8meV,观测到3个特征谱结构,与光致发光结果相一致。     

Optical properties of undoped and iodine-doped CdTe

A comprehensive study of the properties of undoped and iodine-doped CdTe structures by photoluminescence (PL) and photoreflectance (PR) is reported. Undoped bulk CdTe and iodine-doped CdTe layers grown by metalorganic molecular beam epitaxy on (lOO)-oriented CdTe and (211)B-oriented GaAs substrates with electron concentrations ranging from 10¹⁴ to mid-10¹⁸ cm^-3 were included in this study. Lineshape modeling of 80KPL and PR spectra indicated the presence of both free exciton and donor-hole transitions at the higher doping levels. Strong PL and PR signals were also observed at room temperature. If only a single transition is considered for the analysis of the 300K spectra, the PL emission peak and the PR transition energy both exhibit a strong dependence on electron concentration for doped layers. However, lineshape modeling of the room-temperature spectra indicated the presence of multiple transitions consisting of free exciton and direct band-to-band transitions. The use of two transitions resulted in a constant value of bandgap over the entire range of conductivities studied. A strong correlation remained between the broadening of the PR and PL spectra and excess carrier concentration N_D-N_A. In addition, the E₁ transition energy measured by PR was found to vary dramatically with growth conditions.     

光调制反射谱用于研究MBEGaAs1—xSbx／GaAs外延膜

本文讨论了量子阱的光调制反射谱(PR)线形,并采用光调制反射谱研究了MBE GaAs_(1-x)Sb_x/GaAs异质结、应变层量子阱。可见,用PR谱研究MBE外延膜具有一定的优越性。     

Photoreflectance spectroscopy study of vertical cavity surface emitting laser structures

This paper summarises the application of the laser-based electro-absorptive technique of photoreflectance (PR) for the study of vertical cavity surface emitting lasers (VCSELs). PR results are shown to reveal the technologically important cavity mode and ground state quantum well exciton structures. AlGaAs/GaAs based quantum well VCSELs were examined with and without top mirror layers as a function of laser pump excitation conditions, with results compared with angle-dependent PR data. Cavity mode and quantum well alignments were also studied with reference to the un-modulated reflectance signal as well as correlated with photoluminescence data. The results demonstrate the importance of PR metrology for state-of-art VCSEL characterisation.     

应用于1.55 微米的高含氮量GaInNAs/GaAs单量子阱的光激反射和光激发光的研究

用光的反射光谱和光的光致发光光谱的测量对Ga0.69In0.31NxAs1-x/GaAs 的单量子阱的光学特性作了研究,在单量子阱的反射光谱中,观察到 GaAs 能隙之上的 Franz-Keldysh 振荡和来源于量子阱区的各种类激子跃迁,Franz-Keldysh 振荡确定量子阱的内建电场并发现它是随 N 的浓度增加而增加;反射信号随样品中氮耦合增强而减弱,因为温度降低时载流子的定域作用导致调制效应的弱化.激子跃迁的能量和温度关系按照 Varshni 和爱因斯坦一玻司方程作了研究,在 PL 谱中观察到的 11H 跃迁能量和谱线展宽的温度反常关系解释为起源于氮耦合所引起的定域态,这种样品的谱线特征为随氮成份增加出现红移,氮结合作用的另一个结果是晶体的性质严重退化,明显地表现线宽受温度的影响增大.总之,氮引进系统会观察到GaAs 边带以上的 FkO 导致内建场增大,有低温时高激发态叠加并屏闭在定域态上的部分调制外场作用的倾向.PL 峰能量和线宽对温度的反常关系可以理解为由氮的结合作用引起的形成定域态和去除定域态的竞争结果.     

Photoreflectance studies of (Al)Ga- and N-face AlGaN/GaN heterostructures

Photoreflectance studies were carried out on Al_xGa_1−xN/GaN heterostructures confining high-mobility polarization-induced two-dimensional electron gases. By analyzing the Franz-Keldysh oscillations for samples with (Al)Ga- and N-face polarity, we obtained values for the surface electric field up to F=380 kV cm⁻¹ at room temperature. Taking into account spontaneous and piezoelectric polarization, the density of charged surface states and the bare surface potential are estimated. The results unambiguously prove the presence of donor- and acceptor-like surface states for samples with (Al)Ga- and N-face polarity, respectively. A change of the electric field was observed upon the exposure of the surface to a polar liquid, demonstrating the applicability of these structures for chemical sensors.     

Contactless electromodulation and surface photovoltage spectroscopy for the nondestructive, room temperature characterization of wafer-scale III–V semiconductor device structures

This paper will review the use of the contactless methods of photoreflectance, contactless electroreflectance, and surface photovoltage spectroscopy for the nondestructive, room temperature characterization of a wide variety of wafer-scale III–V semiconductor device structures. Some systems that will be discussed include heterojunction bipolar transistors (including determination of collector and emitter doping levels, alloy composition, and dc current gain factor, pseudomorphic GaAlAs/InGaAs/GaAs high electron mobility transistors (including the determination of the composition, width, and two-dimensional electron gas density in the channel), edge emitting lasers [InGaAsP/InP (including the detection of p-dopant interdiffusion), graded index of refraction separate confinement heterostructure GaAlAs/GaAs/InGaAs], and vertical-cavity surface-emitting lasers (determination of fundamental conduction to heavy-hole excitonic transition and cavity mode). These methods are already being used by more than a dozen industries world-wide for the production-line qualification of these device structures.