H. T. Lin

Cathodoluminescence and electron beam induced current study of partially relaxed AlGaAs/GaAs/InGaAs heterojunction phototransistors under operating conditions

We have studied time-resolved cathodoluminescence (CL) and electron beam induced current (EBIC) on AlGaAs/GaAs/InGaAs heterojunction phototransistors under operating conditions, i.e., at room temperature and under bias. Devices from four wafers, with a different amount of lattice relaxation, were tested. It is shown that the CL intensity increases more than one order of magnitude as the voltage is increased and the current gain of the device turns on. The voltage dependence of the CL signal is analogous to the current–voltage curve of the transistor. The buildup in CL intensity was found to be much less in devices with low current gain showing that the CL intensity correlates to the electrical gain of the device. Time resolved CL showed two distinct CL decay times, one very short, a few nanoseconds, and one long, of the order of microseconds. This indicates that two fundamental recombination processes are present, which we attribute to a spatially direct recombination between carriers in the base and a sp...

Carrier relaxation and recombination in GaAs/AlGaAs quantum heterostructures and nanostructures probed with time-resolved cathodoluminescence

We have examined the kinetics of carrier relaxation in GaAs/AlGaAs quantum wells (QWs), quantum wires (QWRs), and quantum boxes (QBs) with time-resolved cathodoluminescence (CL). In the cases of QWRs and QBs, the nanostructures were grown via a size-reducing growth approach on pre-patterned GaAs(001) substrates composed of stripes and mesas, respectively. The growth involved deposition of multiple GaAs/AlGaAs layers in order to establish both structural and optical markers which facilitated the identification of important features in transmission electron microscopy (TEM) and CL experiments. In TEM measurements, the lateral dimensions of the top-most GaAs layers in typical stripe and mesa structures comprising the QWRs and QBs delineate GaAs regions expected to exhibit 2D and 3D quantum confinement effects, respectively. Time-delayed CL spectra of all three structures reveal that the initial capture of carriers in the active regions occurs on a time scale less than the temporal resolution of the CL system...

In-situ fabrication of three-dimensionally confined GaAs and InAs volumes via growth on non-planar patterned GaAs(001) substrates

Three-dimensionally confined GaAs/AlGaAs and InAs/GaAs structures on 〈100〉 oriented square mesas patterned onto GaAs(001) substrates are realized, in-situ, via size-reducing molecular beam epitaxy. Two stages of mesa top pinch-off involving ∼ {103} and subsequently {101} side facets are revealed. GaAs and InAs quantum boxes with lateral linear dimensions down to 40 nm and confined by AlGaAs and GaAs, respectively, are reported. For InAs, the strain relief in mesas is found to enhance the well known ∼ 2 ML thickness for three-dimensional island formation on unpatterned substrates to, remarkably, > 5 ML for mesa size ∼ 75 nm. Cathodoluminescence emission from the InAs on the mesa top attests to its good optical quality.

CATHODOLUMINESCENCE STUDY OF DOMAINS, DEFECTS, AND INTERDIFFUSION IN ZNSE/GAAS(100)

The distribution of dislocations and domains found in thin ZnSe films grown by molecular‐beam epitaxy on GaAs(100) has been examined with low‐temperature cathodoluminescence (CL) imaging and spectroscopy. Dark‐line and bright‐line defects in the low‐temperature CL imaging of the free‐exciton (FE) and Y‐band emissions, respectively, are found to correlate with the presence of [110]‐oriented misfit dislocations for 1‐μm‐thick films found to grow nearly two dimensionally. For a sample exhibiting mixed two‐ and three‐dimensional growth characters, large domains (∼1–5 μm widths) in the CL imaging of the ZnSe FE emission were found to correlate with a cellular pattern found in the imaging of the GaAs exciton and band‐edge–to–acceptor emissions. These results show that the optical properties of the ZnSe film and GaAs substrate are coupled and influenced by Zn diffusion into the substrate during growth.

Linearly polarized and time-resolved cathodoluminescence study of strain-induced laterally ordered (InP)2/(GaP)2 quantum wires

The optical properties of (InP)2/(GaP)2 bilayer superlattice (BSL) structures have been examined with linearly polarized cathodoluminescence (CL), time-resolved CL spectroscopy, and cathodoluminescence wavelength imaging. An In and Ga composition modulation of ∼18% forms during the metalorganic chemical vapor deposition growth of short period (InP)2/(GaP)2 bilayer superlattices. Transmission electron microscopy showed a period of ∼800 A along the [110] direction, resulting in coherently strained quantum wires. A strong excitation dependence of the polarization anisotropy and energy of excitonic luminescence from the quantum wires was found. The results are consistent with a phase-space and band filling model that is based on a k⋅p and two dimensional quantum confinement calculation which takes the coherency strain into account. CL images reveal that defects in the BSL originate from the GaAs substrate and/or the initial stages of InGaP growth. The effects of defects on the band filling, carrier relaxation...

Influence of misfit dislocations on thermal quenching of luminescence in InxGa1−xAs/GaAs multiple quantum wells

The temperature dependence of the cathodoluminescence (CL) originating from In0.21Ga0.79As/GaAs multiple quantum wells has been studied between 86 and 250 K. The CL intensity exhibits an Arrenhius‐type dependence on temperature (T), characterized by two different activation energies. The influence of misfit dislocations and point defects associated with strain relaxation on the thermal quenching of luminescence has been investigated, and the spatial variation in the activation energies has been examined. The CL intensity dependence on temperature for T≲150 K is controlled by thermally activated nonradiative recombination. For T≳150 K the decrease in CL intensity is largely influenced by thermal re‐emission of carriers out of the quantum wells.

INFLUENCE OF DEFECTS ON ELECTRON-HOLE PLASMA RECOMBINATION AND TRANSPORT IN A NIPI-DOPED INXGA1-XAS/GAAS MULTIPLE-QUANTUM WELL STRUCTURE

The nonlinear optical and transport properties of a nipi‐doped InxGa1−xAs/GaAs multiple‐quantum well sample (x=0.23) has been studied using a novel approach called electron‐beam‐induced absorption modulation (EBIA). The absorption in the sample is modulated as a result of screening of the built‐in electric field in the nipi structure due to excess carrier generation. The change in field causes a Stark shift of the first quantized optical transitions in QWs which are situated in the intrinsic layers. In EBIA, a scanning electron probe is used to locally generate an electron–hole plasma that is used to study the spatial distribution of defects that impede excess carrier transport and reduce the lifetime of spatially separated carriers. The Stark shift in the MQW structure is imaged with micrometer‐scale resolution and is compared with cathodoluminescence imaging results which show dark line defects resulting from strain‐induced misfit dislocations. Theoretical calculations using Airy functions in the transf...

Influence of structural defects on carrier recombination and current gain in an InGaAs/AlGaAs/GaAs heterojunction phototransistor

We have studied the influence of structural defects on the spatial variation of radiative and nonradiative recombination in an InGaAs/AlGaAs/GaAs resonant cavity enhanced npn heterojunction phototransistor (HPT) structure using cathodoluminescence (CL) and electron beam‐induced current (EBIC) imaging. Absorber layers of InGaAs/GaAs multiple quantum wells (MQWs) are used to provide the photosensitivity for light with wavelengths that are transparent to the GaAs substrate. The current gain of the HPT under an applied bias voltage depends on the hole accumulation efficiency in the base and therefore on the hole lifetime. Strain relaxation‐induced misfit dislocations in the MQWs are found to create regions of enhanced nonradiative recombination thereby reducing locally the hole accumulation efficiency and current gain. The reduction in the local EBIC signal caused by the dark line defects is less than ∼20%, suggesting that misfit dislocations in this sample have a relatively small impact on overall device per...

Time‐resolved cathodoluminescence study of carrier relaxation in GaAs/AlGaAs layers grown on a patterned GaAs(001) substrate

We have examined the kinetics of carrier relaxation in three‐dimensionally confined GaAs/AlGaAs layers obtained by growth on prepatterned GaAs(001) with time‐resolved cathodoluminescence (CL). Time‐delayed CL spectra at 87 K reveal that (i) relaxation of hot carriers into the largest 3D confined regions occurs on a time scale of a few hundred ps during the onset of luminescence, and (ii) the luminescence decay time also increases for these larger confined regions, owing to thermal reemission from QWs, diffusion across AlxGa1−xAs barriers, and carrier feeding from surrounding thinner QWs.

Ambipolar diffusion anisotropy induced by defects in nipi‐doped In0.2Ga0.8As/GaAs multiple quantum wells

The influence of strain‐induced defects on the ambipolar diffusive transport of excess electrons and holes in the δ‐doped InGaAs/GaAs multiple quantum well system has been examined with a new technique called electron‐beam‐induced absorption modulation (EBIA). The excess carrier lifetime and diffusion coefficient are obtained by a one‐dimensional diffusion experiment that utilizes EBIA. An anisotropy in the ambipolar diffusion along both high‐symmetry 〈110〉 directions is found, and this is seen to correlate with the distribution of dark line defects observed in cathodoluminescence.