B. N. Murdin

Suppression of non-radiative processes in semiconductor mid-infrared emitters and detectors

Abstract We review the methods that have been used for suppressing non-radiative processes in mid-infrared (MIR) semiconductor lasers and detectors. Specifically we discuss the results of techniques that have been used recently to minimise the deleterious effect of Auger recombination processes in interband detectors and (bi-polar) lasers, and of phonon scattering in quantum well photodetectors (QWIPs) and quantum cascade (QC) lasers. After summarising the theory of the suppression of Auger and phonon processes in these devices, sections are devoted to specific III–V, II–VI and lead salt materials systems; further sections are devoted to subband detectors, subband cascade lasers, interband cascade lasers and to non-equilibrium devices from the InSb and HgCdTe systems.

Far‐infrared pump‐probe measurements of the intersubband lifetime in an AlGaAs/GaAs coupled‐quantum well

We report pump‐and‐probe measurements of the electron intersubband lifetime (T1) in an AlGaAs/GaAs heterostructure using a picosecond pulsed far‐infrared laser. The subband spacing (11 meV) is less than the optical‐phonon energy. Time‐resolved measurements yield intersubband lifetimes ranging from T1=1.1±0.2 ns to T1=0.4±0.1 ns depending on measurement conditions. Results are in agreement with previous lifetime measurements on the same sample using continuous excitation at intensities ≤1 W/cm2. The steady‐state measurements yielded shorter lifetimes at high excitation intensities, possibly due to carrier heating leading to intersubband scattering by optical phonon emission.

Optically detected magnetic resonance of deep centers in molecular beam epitaxy ZnSe:N

Optically detected magnetic resonance has been used to investigate the deep level recombination processes in p‐type ZnSe grown by molecular beam epitaxy and doped with nitrogen. In addition to the well‐known shallow donor resonance at g=1.11, an anisotropic deep donor resonance is observed with g=1.38 and a deep acceptor resonance is detected at g=2. These results are consistent with the pair recombination processes proposed by us previously where the compensating deep donor was assigned to the VSe‐Zn‐NSe complex.

IDENTIFICATION OF VSE-IMPURITY PAIRS IN ZNSE:N

Optically detected magnetic resonance (ODMR) of heavily nitrogen‐doped ZnSe at 24 GHz reveals an anisotropic spectrum which can be assigned to a deep donor comprised of a Se vacancy paired with an impurity. The newly resolved spectrum is trigonal along 〈111〉 with g∥=2.0072(2) and g⊥=2.0013(2). Comparison of this g tensor with previous work leads to the assignment of the spectrum to VSe‐X, where X is most probably Cu or Ag. Combining this result with previous photoluminescence and ODMR work shows that ZnSe:N has three distinct donors which include both impurities and intrinsic defects.

Auger recombination dynamics of Hg0.795Cd0.205Te in the high excitation regime

A direct measurement of carrier recombination, far from equilibrium, in Hg0.795Cd0.205Te (Nd−Na=3.3×1014u2009cm−3) has been made on a picosecond time scale with a pump–probe technique using a free-electron laser. Over the range of carrier densities (5×1016–3×1017u2009cm−3) and at the temperatures (50–300 K) studied experimentally, contributions to the recombination from Auger, Shockley–Read–Hall, and radiative mechanisms were calculated using an analytic approximation, with carrier degeneracy included, Auger-1 (CCCH) recombination rates were calculated, which also gave the Auger-7 (CHHL) rates via a simple relationship. Excellent agreement was obtained, with Auger-1 dominant at all temperatures and, significantly, for T>225u2009K when the sample is intrinsic, the Auger-7 contribution was found to be important.

Key issues for mid–infrared emission

Semiconductor emitters in the mid–infrared (mid–IR) fall broadly into two categories; interband and intersubband devices. The former are based on transitions from conduction to valence bands, whereas the latter use one–dimensionally quantum confined states within the conduction band. The problems which limit light–emitting diode (LED) and laser operation by keeping efficiencies low and laser thresholds high are quite different in these two cases, but both stem from high competing transition rates due to non–radiative processes. In interband devices Auger scattering dominates, and in intersubband devices, though Auger scattering is still fast, phonon emission is even faster. Working intersubband devices are only made possible by the fact that the phonon scattering is strongly resonant in energy, so three–level laser schemes may be made with a dynamic equilibrium that is inverted. We shall review our own work on suppression of these non–radiative processes by band–structure engineering, and we present new results on Auger suppression in 11 μm bandgap InSbN alloys and predictions for phonon suppression in narrow–gap intersubband devices.

Compensation processes in molecular beam epitaxially grown zinc selenide doped with nitrogen

Abstract Recent work has shown that nitrogen produced in a plasma source is a p-type dopant in MBE grown ZnSe with N a − N d to 1×10 18 cm -3 , but at these concentrations the material is highly compensated. In a previous study, we have examined the PL spectra of nitrogen doped material grown in our laboratory and have shown that there are two sets of donor-acceptor pair (DAP) peaks which can be explained by a simple model involving a nitrogen acceptor and two donors. The first donor is a native shallow donor and the second is a nitrogen related compensating donor thought to be a complex of the form V Se -Zn-N Se . Optically detected magnetic resonance results on samples showing both shallow and deep DAP luminescence show signals due to the shallow isotropic donors and deep anisotropic donors consistent with our proposed model. Calculations of the vacancy concentrations and degree of compensation that should be expected in nitrogen doped ZnSe show that at all temperatures and under all growth conditions the material is highly undersaturated with vacancies. The barriers operating to prevent the compensation are discussed.

Complete bleaching of the intersubband absorption in GaAs/AlGaAs quantum wells using a far‐infrared free‐electron laser

The intensity‐dependent intersubband absorption in GaAs/AlGaAs quantum wells with a subband separation smaller than the optical phonon energy has been measured with a pulsed far‐infrared free‐electron laser. Complete bleaching of the absorption is observed at I=200 kW/cm2. Fitting the data with a two‐level system yields a characteristic time constant of 1–2 ps. Possible interpretations, considering the finite pulse width of the laser, are discussed.

Direct determination of Shockley-Read-Hall trap density in InSb/InAlSb detectors

Accurate determination of trap density in the active region of mid-infrared narrow-bandgap detectors is crucial in the development towards background-limited performance at higher operating temperatures. We have used both optical and electrical measurements to determine the trap density in InSb/InAlSb nonequilibrium detector structures. Both of these techniques result in very good agreement with trap densities of 5×1014xa0cm-3.

Saturation absorption studies of intersubband relaxation rates in a p-GaAs/AlGaAs QW using a free electron laser

We present saturation absorption studies of the HH1 --> LH1 intersubband transition in a 70 Angstrom p-type GaAs/AlGaAs multi-quantum well. The subband separation in this sample, with strongly non-parabolic bands, varies from 26 meV at k = 0 to 16 meV at the Fermi wave vector. The absorption behaviour of this transition is studied over a range of four orders of magnitude of intensity of incident THz radiation. The results are analysed within the contexts of a simple two-level model and an instantaneous thermalization model. The results indicate that the lifetime obtained with these models is limited by the pulsewidth, i.e. is greater than 1.7 ps. This places a lower limit on the energy relaxation lime for transitions below the LO-phonon energy