Showing papers by "Hongen Shen published in 2013"

Investigation of Trap States in Mid-Wavelength Infrared Type II Superlattices Using Time-Resolved Photoluminescence

Abstract: Time-resolved photoluminescence (TRPL) spectroscopy is used to study the minority-carrier lifetime in mid-wavelength infrared, n-type, InAs/Ga1−xInxSb type II superlattices (T2SLs) and investigate the recombination mechanisms and trap states that currently limit their performance. Observation of multiple exponential decays in the intensity-dependent TRPL data indicates trap saturation due to the filling then emptying of trap states and different Shockley–Read–Hall (SRH) lifetimes for minority and majority carriers, with τmaj (τn0) ≫ τmin (τp0). Simulation of the photoluminescence transient captures the qualitative behavior of the TRPL data as a function of temperature and excess carrier density. A trap state native to Ga1−xInxSb is identified from the low-injection temperature-dependent TRPL data and found to be located below the intrinsic Fermi level of the superlattice, approximately 60 ± 15 meV above the valence-band maximum. Low-temperature TRPL data show a variation of the minority-carrier SRH lifetime, τp0, over a set of InAs/Ga1−xInxSb T2SLs, where τp0 increases as x is varied from 0.04 to 0.065 and the relative layer thickness of Ga1−xInxSb is increased by 31%.

Solar-blind AlxGa1−xN/AlN/SiC photodiodes with a polarization-induced electron filter

Abstract: Heterogeneous n-III-nitride/i-p silicon carbide (SiC) photodetectors have been demonstrated that enable the tailoring of the spectral response in the solar blind region below 280 nm The negative polarization induced charge at the aluminum gallium nitride (AlxGa1−xN)/aluminum nitride (AlN) interface in conjunction with the positive polarization charge at the AlN/SiC interface creates a large barrier to carrier transport across the interface that results in the selective collection of electrons photoexcited to the Γ and L valleys of SiC while blocking the transport of electrons generated in the M valley In addition, the AlxGa1−xN alloys act as transparent windows that enhance the collection of carriers generated by high energy photons in the fully depleted SiC absorption regions These two factors combine to create a peak external quantum efficiency of 76% at 242 nm, along with a strong suppression of the long-wavelength response from 260 nm to 380 nm

Time-resolved photoluminescence study of carrier recombination and transport in type-II superlattice infrared detector materials

Abstract: Time-resolved photoluminescence (TRPL) is used to study the minority carrier lifetime in type-II superlattice (T2SL) infrared detector materials to investigate the recombination mechanisms, trap states and transport properties that currently limit their performance. Measurements of carrier lifetime have shown that InAs/Ga1-xInxSb T2SLs are dominated by non-radiative Shockley-Read-Hall (SRH) recombination, resulting in short minority carrier lifetimes (10’s of nanoseconds at 77 K). A trap energy of ~60 meV above the valence band is identified in mid-wavelength infrared n-type InAs/Ga1-xInxSb T2SLs, where trap saturation (non-exponential decay) is observed under high injection levels due to a significantly faster hole capture rate than electron capture rate. Lifetime measurements in “Ga-free” InAs/InAs1-xSbx T2SLs exhibit an order-of-magnitude increase in lifetime (100’s of nanoseconds at 77 K) with contributions from both radiative and non-radiative recombination. This improvement is attributed to the reduction of non-radiative recombination centers from the superlattice with the elimination of Ga and suggests that the SRH trap(s) limiting the carrier lifetime of InAs/Ga1-xInxSb T2SLs is native to the Ga1-xInxSb layer. Additionally, radiative recombination is observed in an InAs/GaSb T2SL using a sub-bandgap CW laser to saturate the SRH recombination centers, yielding a radiative lifetime of ~140 ns at 77 K. Since carrier transport is a concern in Ga-free T2SLs, it is investigated by studying samples grown with and without barriers (to contain injected carriers to the absorber region). It is determined that carrier transport is poor in InAs/InAs1-xSbx T2SLs because negligible differences are observed in the carrier lifetime.

Wavelength stable, p-side-down green light emitting diodes grown by molecular beam epitaxy

Abstract: p-side-down, single heterostructure n-InGaN/p-GaN light emitting diodes grown by molecular beam epitaxy exhibited stable peak emission wavelengths as long as 550 nm for current densities in excess of 100 A/cm2, and minimal efficiency droop up to 150 A/cm2 without the use of an electron blocking layer. This behavior is consistent with the formation of a two-dimensional hole gas in the n-InGaN layer and a higher barrier to electron overflow in the conduction band due to the negative polarization charge at the n-InGaN/p-GaN interface.

Optimization of Annealing Process for Improved InGaN Solar Cell Performance

Abstract: We report enhanced performance of InGaN solar cells grown by metalorganic chemical vapor deposition through optimization of the annealing of the epitaxial wafer before device fabrication. We varied the annealing environment gas mixtures as well as temperatures to obtain the optimized annealing condition. It was found that the major improvement of the nitride solar cell efficiency after annealing is in the increase of the Voc. In addition, annealing at the reasonably moderate temperature of 550°C in O2 environment results in the highest-efficiency InGaN solar cell devices compared with devices annealed at different temperatures and in different gas environments.

Study of temperature-dependent carrier transport in a p-GaN/i-InGaN/n-GaN solar cell heterostructure using ultrafast spectroscopy

Abstract: Temperature-dependent carrier transport is investigated using ultrafast spectroscopy in a p-GaN/i-InGaN/n-GaN solar cell with heavily-doped layers to compensate for polarization charges at the hetero-interface. We observe a flip in the transport direction at 110 K.

Terahertz studies of carrier localization in spontaneously forming polar lateral heterostructures

Abstract: We employ near-bandgap terahertz emission spectroscopy to study lateral heterostructures resulting from basal plane stacking faults in m -plane GaN. The predominant stacking faults have I1 character and behave as an array of spontaneously forming layers comprised of a single cubic stacking sequence within the wurtzite matrix that terminate the spontaneous polarization along the in-plane c-axis, leading to strong lateral electric fields. Spectral tuning of femtosecond excitation pulses enables observation of the transition from carrier transport in the continuum to formation of instantaneous dipoles and nonlinear susceptibility associated with both the quantum-well-like regions of the cubic layers and polarization-induced triangular-like potentials. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)