sem 2 exam 16 Flashcards
What is the probability of a quantum state being occupied by an electron when E = Ef?
1/2 regardless of temperature
Why is the photon absorption coefficient proportional to √(hν - Eg)?
Because of density of states (3D)
For a semiconductor at the end of the derivation of the 3D density of states function, √E becomes what?
√E ⇒ √(E -Eg)
How accurate is the infinite well approximation for the first 3 energy levels?
It’s a reasonable approximation for E1, doubtful for E2 and not valid for E3
What is the general solution for the time independent Schrodinger equation when V(x) = 0 ?
ψ(x) = Asin(kx) + Bcos(kx)
What is x when the wavefunction is continuous?
When ψ is continuous, x = 0.
For a Dirac comb potential, what is potential V(x) given by at x=0 ?
−αδ(x)
Why does g(E)δ(E) = n(k)δk ?
Because of the differential quotient.
What are the features of the Dirac z-function?
|cos(ka)| has to be < 1 and therefore leads to forbidden and allowed regions known as energy gaps and bands respectively. This is characteristic of periodic solids such as semiconductors.
What is the separation of the super lattice of quantum wells used for a quantum well laser?
10 nm
How is a population inversion achieved for a quantum well laser?
Electrical injection of carriers traps them in the quantum wells, quantum mechanical tunnelling happens at a faster rate than recombination.
When does lasing occur for a quantum well laser?
When the wells are in a suitable optical cavity with aperture.
How do you show that a set of states cannot be cloned?
Demonstrate they aren’t orthogonal.
When are two states orthogonal?
When their inner product is equal to zero.
What does a CNOT gate do in terms of its input and output?
When |A> IS 0, B remains the same.
When |A> is 1, B is inverted. A always remains the same.
When the input of a Hadamard gate is |0 0>, what is the output?
(|0 0> + |1 1>)/√2 = |β00>
When the input of a Hadamard gate is |0 1>, what is the output?
(|0 1> + |1 0>)/√2 = |β01>
When the input of a Hadamard gate is |1 0>, what is the output?
(|0 0> - |1 1>)/√2 = |β10>
When the input of a hadamard gate is |1 1>, what is the output?
(|0 1> - |1 0>)/√2 = |β01>
Why is reduced dimensionality preferable for optoelectronics devices in terms of the density of states?
In general, the number of states near the band edge increases significantly for reduced dimensionality and hence potential for better device performance.
How can a 2D electron gas be created in a quantum well, using a GaAs/AlGaAs semiconductor heterostructure?
A 2D electron gas can be created at the interface between doped (n-type) and intrinsic GaAs. Therefore using n-AlGaAs/GaAs/n-AlGaAs creates a quantum well in which electrons become trapped.
If the barrier is 300 meV, is the infinite well model more valid for electrons or holes?
It is more valid for heavy holes than for electrons and light holes.
How does the dispersion relation of a free electron differ from that of a quantum well?
The free electron follows a path that is not quantised as the infinite square well is.
What is a bell state?
A bell state is when two qubits are entangled and can be given by:
(|0 1> +|1 0>)/√2
