Picosecond strain pulses probed by the photocurrent in semiconductor devices with quantum wells

Ultrafast acoustic wave packets are detected by probing the photocurrent in semiconductor devices containing quantum wells. The strain pulses were generated by thermalization of a femtosecond laser pulse in a thin metal film deposited on the surface of the GaAs substrate opposite to the semiconductor device. The probing was realized by measuring the photocurrent excited by a femtosecond optical pulse with photon energy close to the excitonic resonance of the quantum well. Two types of devices are used: a reverse biased (AlGa)As p-i-n tunneling diode containing a GaAs quantum well in its intrinsic region and a planar device containing an (InGa)As quantum well. The change in photocurrent arises from the strain-induced shift of the quantum well excitonic resonance due to deformation potential electron-phonon coupling. The method has a picosecond temporal resolution, shows a high sensitivity to subterahertz acoustic wave packets and has potential for ultrafast control of electrical conductance in semiconductor devices.

Figure 1

Diagramatic representation of the sensitivity of the photocurrent $I_p$ in a semiconductor QW to strain $\eta$ (piezospectroscopic effect). $I_0$ and Δ${}_0$ are the stationary photocurrent and detuning respectively, present when $\eta$ $=$ 0.

Figure 2

(a) The scheme of layers in the p-i-n device including a QW. (b) $\mathit{IV}$ characteristics of the device under illumination from a cw laser with various wavelengths and in the dark (dashed line). (c) The band diagram of the p-i-n device.

Figure 3

(a) The temporal profile of the strain pulse generated in the Al film and injected into the GaAs substrate. (b) The experimental scheme for strain-pulse detection in the p-i-n device.

Figure 4

(a) Structural schematic of the lateral device sample which includes two QWs. (b) The photoluminescence spectrum of the sample. (c) The photocurrent spectrum measured for cw excitation. (d) $\mathit{IV}$ characteristic of the device when the QW, situated closest to the surface, is optically excited by a cw laser.

Figure 5

Photocurrent changes as a function of the delay between pump and probe measured in the p-i-n device for $\hslash \omega$ $=$ 1.56 eV (λ $=$ 795 nm), $V_b$ $=$ 3.2 V, pump density 55 μJ/cm${}^2$, average probe power density 500 W/cm${}^2$. The value $t_0$ $=$ 0 corresponds to probing when the middle of the incident strain pulse passes the QW.

Figure 6

Photocurrent changes $\Delta I_p\left(t_0\right)$ which occur when the incident strain pulse passes the p-i-n device QW for various probe photon energies $\hslash \omega$, with $W$ $=$ 60 μJ/cm${}^2$ and probe power density 100 W/cm${}^2$.

Figure 7

Photocurrent changes $\Delta I_p\left(t_0\right)$ when the incident strain pulse is passing the QW of the p-i-n device for various reverse bias voltages $V_b$, with $\hslash$ω $=$ 1.56 eV, W $=$ 60 μJ/cm${}^2$ and probe power density 100 W/cm${}^2$.

Figure 8

(a) Photocurrent changes $\Delta I_p\left(t_0\right)$ when the incident strain pulse is passing the QW of the p-i-n device for various pump densities $W$ at the Al film, $V_b$ $=$ 3 V, $\hslash$ω $=$ 1.56 eV. (b) The dependence of the changes of photocurrent amplitude $\Delta I_0$ on pump density $W$; the solid line is the linear fit.

Figure 9

Relative photocurrent changes $\Delta I_0/I_0$ measured in p-i-n device as a function of reverse bias voltage and various probe excitation wavelengths λ (photon energies $\hslash \omega$).

Figure 10

Photocurrent changes as function of delay time, measured in the lateral device with two QWs: (a) $\hslash \omega$ $=$ 1.43 eV (λ $=$ 865 nm), only wide QW is excited; (b) $\hslash \omega$ $=$ 1.44 eV (λ $=$ 860 nm), both QWs are excited by the probe beam.

Figure 11

Calculated dependencies of the sensitivity, $\alpha$, of the p-i-n device to strain as a function of reverse bias for three different probe excitation energies with a spectral width of 20 meV and where (a) only interband electron-hole transitions are taken into account; (b) exciton transitions are included in absorption spectrum. Insets show the corresponding spectral profiles, $A_0$($E$), of the optical absorption.