Babak Imangholi

Effects of epitaxial lift-off on interface recombination and laser cooling in GaInP∕GaAs heterostructures

Photoluminescence of GaAs passivated with GaInP is studied over the temperature range 7–450K. Different photocarrier recombination mechanisms are identified as the temperature changes. An interface recombination velocity of less than 0.6cm∕s is measured at 300K. Lift-off processing inhibits but does not preclude laser cooling of GaAs.

Differential luminescence thermometry in semiconductor laser cooling

We demonstrate a non-contact, spectroscopic technique to measure the temperature change of semiconductors with very high precision. A temperature resolution of less than 100 μK has been obtained with bulk GaAs. This scheme finds application in experiments to study laser cooling of solids. We measure a record external quantum efficiency of 99% for a GaAs device.

Precision, all-optical measurement of external quantum efficiency in semiconductors

External quantum efficiency of semiconductor photonic devices is directly measured by wavelength-dependent laser-induced temperature change (scanning laser calorimetry) with very high accuracy. Maximum efficiency is attained at an optimum photo-excitation level that can be determined with an independent measurement of power-dependent temperature or power-dependent photoluminescence. Time-resolved photoluminescence lifetime and power-dependent photoluminescence measurements are used to evaluate unprocessed heterostructures for critical performance parameters. The crucial importance of parasitic background absorption is discussed.

Advances in laser cooling of semiconductors

Laser cooling in semiconductor structures due to anti-Stokes luminescence is reviewed. Theoretical background considering luminescence trapping and red-shifting, the effect of free carrier and back-ground absorption, Pauli band-blocking, and the temperature-dependence of various recombination mechanisms are discussed. Recent experimental results demonstrating record external quantum efficiencies (EQE) in GaAs/GaInP heterostructures are described, and conditions favorable for the first observation of laser cooling in semiconductors are discussed.

Laser cooling of infrared sensors

We present an overview of laser cooling of solids. In this all-solid-state approach to refrigeration, heat is removed radiatively when an engineered material is exposed to high power laser light. We report a record amount of net cooling (88 K below ambient) that has been achieved with a sample made from doped fluoride glass. Issues involved in the design of a practical laser cooler are presented. The possibility of laser cooling of semiconductor sensors is discussed.

99% External quantum efficiency from a GaAs heterostructure at 100 K

Record external quantum efficiency (99%) is obtained for a GaAs/InGaP heterostructure bonded to a dome lens at 100 K. This was measured using a differential luminescence thermometry technique with temperature resolution ~ 30 muK.

Phase fluorometry for semiconductor lifetime measurement

Understanding and quantifying nonradiative recombination is a critical factor for the successful laser cooling of semiconductors. The usual approach to measuring the nonradiative lifetime employs pulsed photoexcitation and monitors the luminescence decay via time-resolved photon counting. We present an alternative approach that employs phase fluorometry with a lock-in amplifier. A sinusoidally modulated diode laser is used for excitation. Lifetime data are extracted from the frequency dependent phase shift and amplitude response of the photolumi-nescence signal, detected by a photomultiplier tube. Samples studied include high quality AlGaAs/GaAs/AlGaAs and GaInP/GaAs/GaInP double heterostructures, grown by MBE and MOCVD. Data over a temperature range from 10 to 300 K is compared with results obtained in time-domain measurements.

Heterostructure design optimization for laser cooling of GaAs

Doping of the clad layers in thin GaAs/GaInP heterostructures, displaces the band energy discontinuity, modifies the carrier concentration in the active GaAs region and changes the quality of the hetero-interfaces. As a result, internal and consequently external quantum efficiencies in the double heterostructure are affected. In this paper, the interfacial quality of GaAs/GaInP heterostructure is systematically investigated by adjusting the doping level and type (n or p) of the cladding layer. An optimum structure for laser cooling applications is proposed.