D. G. Liu

Franz–Keldysh oscillations of δ‐doped GaAs

Si‐δ‐doped GaAs (N2D ≊ 1011 cm−2) samples grown by molecular‐beam epitaxy are investigated by using photoreflectance spectroscopy. The oscillations observed above the GaAs fundamental band gap are attributed to the Franz–Keldysh effect in the region between the δ‐doped layer and the crystal surface. This ascription is confirmed by detailed studies through varying the cap thickness (250–2500 A), temperature (10–450 K), and laser pump power (0.05–7 mW/cm2). The surface potential deduced from the Franz–Keldysh oscillations is found to be temperature and laser pump power dependent, which is explained by taking the surface photovoltaic effect into account. The surface Fermi level has been measured by this method and is found to have the value 0.73±0.02 V.

Delta-doped quantum well structures grown by molecular beam epitaxy

Delta doping in quantum well structures has been studied. The quantum wells consist of a strained InGaAs layer sandwiched between two GaAs layers. The layers were undoped except for a sheet of Si dopants deposited in the middle of the quantum well. Structures with various doses and quantum well thicknesses were studied and compared. Capacitance voltage measurements were carried out to determine the carrier distribution. A very narrow carrier profile with a full width at half maximum of only 12 A has been achieved. This is the narrowest carrier profile ever reported for any growth technique.

Resonant tunneling of electrons from quantized levels in the accumulation layer of double-barrier heterostructures

We report the first observation of the resonant tunneling features associated with the quantized levels in the accumulation layer of the double‐barrier resonant tunneling structure (DBRTS) with undoped electrodes. This quantum effect causes additional kinks in the current‐voltage (I‐V) characteristic and an increasingly enhanced oscillation behavior in the differential conductance‐voltage (G‐V) curve. Three discrete quantum levels have been observed based on the room‐temperature G‐V curve. Our measurements are made without the presence of magnetic field and thus the experimental results are totally different from the magneto‐oscillation.

Precise determination of aluminum content in AlGaAs

The Al composition of AlGaAs has been determined by four methods: high‐resolution transmission electron microscopy (HRTEM), reflection high‐energy electron diffraction (RHEED), photoluminescence (PL), and double‐crystal x‐ray diffraction (DCXRD). HRTEM is direct and the most accurate method because it does not involve any formula or extrapolation. Using the result obtained from HRTEM as a standard, we have calibrated the results from other methods. RHEED intensity oscillation is found to be accurate and reliable, if the growth conditions are correctly chosen. Comparing the PL results with those determined from HRTEM and RHEED, we suggest three formulas to determine the Al contents at different temperatures. We also proposed a polynomial to determine the Al concentration using the DCXRD measurement.

Photoreflection study on the surface electric field of delta‐doped GaAs grown by molecular beam epitaxy

Photoreflectance spectroscopy has been used to study the surface electric‐field strength and the surface potential of delta‐doped GaAs. Franz–Keldysh oscillations in the reflectance spectra were clearly observed and the oscillation periods were used to calculate the internal electric fields of the delta‐doped samples. Based on the measured results and the self‐consistent calculation, a surface potential of 0.6 eV is obtained.

Direct observation of Si delta‐doped GaAs by transmission electron microscopy

Direct observation of the Si delta‐doped layer in GaAs has been achieved by high resolution transmission electron microscopy. Samples with different Si doses, from half a monolayer to two monolayers, were studied. The observed spreading of the delta‐doped layer showed that Si atoms are largely confined in five monolayers at most (in the highest dose case), indicating excellent confinements of dopants in GaAs. From the images, the Si atoms were uniformly distributed in the doped layer, no cluster formation was observed. For delta‐doped GaAs grown at low temperature (480u2009°C), stacking faults originated from the doped layers were observed. These faults were thought to be caused by the large unrelaxed strain in the low‐temperature grown GaAs.

TRANSMISSION ELECTRON-MICROSCOPY STUDY OF HEAVILY DELTA-DOPED GAAS GROWN BY MOLECULAR-BEAM EPITAXY

N‐type and p‐type delta‐doped GaAs grown by molecular beam epitaxy with rather significantly high doses of Si and Be have been investigated by transmission electron microscopy (TEM). The amount of doses ranged from half a monolayer to two monolayers. The microscopic structures of the delta‐doped regions and the adjacent epilayers were directly observed by TEM. The effect of impurity spreading on the heterointerfaces and superlattices was also studied. Si atoms present in Si delta‐doped samples were confined to within a few atomic layers. The Be atoms present in Be delta‐doped samples, however, spread over a quite wide region and caused rough heterointerfaces and wavy superlattices to form. Spreading of Be was attributed to segregation and diffusion which occurred during growth. Stacking faults were found in the delta‐doped samples when they were grown at low temperatures. They could be attributed to local strain caused by heavy doping.

Improved AlGaAs/GaAs double-barrier resonant tunneling structures using two-dimensional source electrons

We report the enhancement of peak‐to‐valley current ratios (PVCRs) of double‐barrier resonant tunneling structures (DBRTSs) based on the AlGaAs/GaAs material system. The PVCRs as high as 25.4 and 18 have been obtained at 77 K for superlattice and alloy barrier structures with 0.2‐μm undoped electrodes, respectively. These are the largest PVCRs to date for AlGaAs/GaAs DBRTSs. The large band bending across the undoped electrodes causes size quantization of the accumulation layer, resulting in better resonant tunneling characteristics.

Influence of indium doping on AlGaAs layers grown by molecular beam epitaxy

Influence of indium doping on the qualities of AlGaAs layers grown by molecular beam epitaxy has been studied. It was found that a proper amount of In doping can increase the photoluminescence intensity drastically by a factor greater than 10 indicating an improvement in the optical quality of AlGaAs epilayers. The improvement in the material quality is attributed to a higher surface migration rate of In atoms than those of Ga and Al atoms leading to a reduction of group III vacancies. However, too great a concentration of In atoms leads to effects that may degrade the film quality.

A novel technique for low‐threshold and high‐power InGaAs/GaAs strained‐layer 0.98‐μm buried heterostructure laser fabrication

A novel fabrication technique has been developed for InGaAs/GaAs strained‐layer buried heterostructure lasers. Dielectric masks and Zn diffusion are not required in this technique. This novel fabrication process is much easier than the conventional approach and yields excellent laser results. A low threshold of 3 mA and high‐power operation for lasing wavelength of 9800±20 A have been achieved with graded index separate confinement heterostructure devices using this novel technique.