O. V. Konstantinov

Temperature dependence of the photoelectric conversion quantum efficiency of 4H–SiC Schottky UV photodetectors

Ultraviolet Schottky photodetectors based on n-4H–SiC (Nd − Na = 4 × 1015 cm−3) epitaxial layers of high purity have been fabricated. Their spectral sensitivity range is 3.2–5.3 eV peaking at 4.9 eV (quantum efficiency is about ~0.3 electron/photon), which is close to the bactericidal ultraviolet radiation spectrum. The temperature dependence of the quantum efficiency of 4H–SiC Schottky structure has been investigated to determine the temperature stability and the mechanism of the photoelectric conversion process. At low temperatures (78–175 K) the quantum efficiency increases with increasing temperature for all photon energy values and then tends to saturate. We suppose that some imperfections in the space-charge region act as traps that capture both photoelectrons and photoholes. After some time the trapped electron–hole pairs recombine due to the tunnelling effect. At high temperatures (more than 300 K), the second enhancement region of the quantum efficiency is observed in the photon energy range of 3.2–4.5 eV. It is connected with a phonon contribution to indirect optical transitions between the valence band and the M-point of the conduction band. When the photon energy is close to a direct optical transition threshold this enhancement region disappears. This threshold is estimated to be 4.9 eV. At photon energies more than 5 eV a drastic fall of the quantum efficiency has been observed throughout the temperature interval. We propose that in this case the photoelectrons and photoholes are bound to form hot excitons in the space-charge region due to the Brillouin zone singularity, and do not contribute to the following photoelectroconversion process.

Peculiarities in the mechanism of current flow through an ohmic contact to gallium phosphide

The temperature dependence of the electric resistance of the In-GaP ohmic contact has been studied in the range from 77 to 420 K. The resistance was measured in GaP plates of various thickness with two In ohmic contacts. The measured ohmic contact resistance increases with temperature in the interval from 230–420 K. It is suggested that the In-GaP ohmic contact is formed by metallic shunts appearing upon deposition of In atoms on dislocations and other imperfections present (with a density evaluated at (4.5–8)×107 cm−2) in the subsurface region of the semiconductor.

The mechanism of current flow in an alloyed In-GaN ohmic contact

The resistance of alloyed In-GaN ohmic contact is studied experimentally. In the temperature range 180–320 K, the resistance per unit area increases with temperature, which is typical of metallic conduction and disagrees with current flow mechanisms associated with thermionic, field-effect, or thermal field emission. It is assumed that In-GaN ohmic contact is formed by conducting shunts arising due to precipitation of In atoms on dislocations. As determined from the temperature dependence of the contact resistance, the number of shunts per unit contact area is ∼(107–108) cm−2, which is close to the dislocation density of 108 cm−2 measured in the initial material.

Dependence of the mechanism of current flow in the in-n-GaN alloyed ohmic contact on the majority carrier concentration

Based on the study of the temperature dependence of resistance of the In-n-GaN alloyed ohmic contacts, it is found that the mechanism of current flow in them substantially depends on the concentration N of uncompensated donors in GaN. At N = 5 × 1016 − 1 × 1018 cm−3, current mainly flows along the metallic shunts, and at N ⩾ 8 × 1018 cm−3 it flows by tunneling.

Mechanism of the current flow in Pd-(heavily doped p-AlxGa1−xN) ohmic contact

The physical mechanism of the current flow in Pd-(heavily doped p-AlxGa1−xN) ohmic contact is studied. Chloride-hydride epitaxy was used to grow the p-Al0.06Ga0.94N solid solution with uncompensated acceptor concentration Na–Nd ranging from 3×1018 up to 1019 cm−3. Thermal vacuum deposition and subsequent thermal treatment were used to form an ohmic Pd contact. It is shown that, after the thermal treatment, the Pd-p-Al0.06Ga0.94N barrier contact with a potential barrier height of about 2.3 V becomes ohmic and the barrier height decreases to approximately 0.05 V. For uncompensated acceptor concentration Na–Nd=3×1018 cm−3, thermionic emission is found to be the main mechanism of the current through the Pd-p-Al0.06Ga0.94N ohmic contact. An increase in Na–Nd to approximately 1019 cm−3 in the solid solution leads to a transition from thermionic emission (at high temperatures) to tunneling (at low temperatures).

The tail of localized states in the band gap of the quantum well in the In0.2Ga0.8N/GaN system and its effect on the laser-excited photoluminescence spectrum

It is established experimentally that the peak in the photoluminescence spectrum of the In0.2Ga0.8N/GaN heterostructure with a quantum well shifts by ∼150 meV as the power density of a nitrogen laser used for excitation is increased from 10 to 1000 kW/cm2. The large blue shift is interpreted as a manifestation of the tail of the density of localized states in the band gap of the quantum well. It is shown that, in the model of the ideal quantum well in which there is no tail of localized gap states, it is impossible to account for the large blue shift. A phenomenological expression is suggested for the density of states. The expression involves the adjustable parameter, namely, the Urbach energy that characterizes the tails of the density of localized states. By this means, both the long-wavelength edge of the luminescence spectrum and the large blue shift can be described. The quasi-Fermi level of photoelectrons depends on the pump intensity and is selected to fit each experimental curve. A qualitative agreement between the theoretical and experimental spectra is obtained, demonstrating that the model is adequate. From the condition for electrical neutrality, the surface carrier concentration is determined. It is found to be several orders of magnitude higher than the pyroelectric concentration in narrow quantum wells.

Light Emission by Semiconductor Structure with Quantum Well and Array of Quantum Dots

A new type of composite active region of a laser, which contains an In0.2Ga0.8As quantum well (QW) and an array of InAs quantum dots (QDs) embedded in GaAs is studied. The QW acts as accumulator of injected carriers, and the QD array is the emitting system located in tunneling proximity to the QW. A theory for the calculation of electron and hole energy levels in the QD is developed. Occupation of the QDs due to the resonance tunneling of electrons and holes from the QW to the QD is considered; the conclusions are compared with the results obtained in studying an experimental laser with a combined active region.

Mechanism of current flow in alloyed ohmic In/GaAs contacts

A mechanism of current flow in an alloyed ohmic In contact to low-doped gallium arsenide (n = 4 × 1015 cm−3) is studied. From the temperature dependence of the contact resistance per unit surface area, it is found that the basic mechanism of current flow is thermionic emission through a potential barrier 0.03 eV in height.

Temperature dependence of the quantum efficiency of 4H-SiC-based Schottky photodiodes

Using metal-semiconductor structures based on a pure epitaxial layer of n-4H-SiC (Nd− Na=4×1015 cm−3), UV photodetectors were created with a maximum photosensitivity at 4.9 eV and a quantum efficiency up to 0.3 el/ph. The photosensitivity spectrum of the base structure is close to the spectrum of bactericidal action of the UV radiation. For photon energies in the 3.4–4.7 eV range, the quantum efficiency of the photoelectric conversion exhibits rapid growth with the temperature above 300 K, which is explained by the participation of photons in indirect interband transitions. This growth is not manifested when the photon energy is close to the threshold energy of direct optical transitions in the nondirect-bandgap semiconductor, which allows the threshold energy to be evaluated (∼4.9 eV).

Increasing Laser Excitation Power Induces Large Blue Shift of the Photoluminescence Peak of Quantum Wells in Gallium Nitride

It is experimentally demonstrated that, as the output power density of a nitrogen laser is increased from 10 to 1000 kW/cm2, a peak in the photoluminescence spectrum of quantum wells (QWs) in GaN shifts by approximately 150 meV. This behavior cannot be interpreted within the framework of the ideal QW model. The observed phenomenon is theoretically explained by the presence of a “tail” in the localized density of states in the QW bandgap and by the filling of bands in the QW by nonequilibrium photogenerated charge carriers. A phenomenological expression for the density of states is proposed, which takes into account the tail in the localized density of states and provides qualitative agreement between theoretical and experimental spectra.