LW4 Lecture Week 4-1 Heterojunctions Fabrication and characterization of p-n junctions 1

2 Heterojunctions: Single heterojunction Fig. 36. Energy band diagram for a p-n heterojunction. Fig. 35. Energy band diagram per above calculations. N-p heterojunction Single heterojunctions: Energy band diagrams for N-AlGaAs – p-GaAs and P- AlGaAs/n-GaAs heterojunctions under equilibrium

3 Energy band diagram: Double Heterojunction Fig. 39. A forward biased NAlGaAs-pGaAs- PAlGaAs double heterojunction diode. Fig. 42 Energy band diagram of a NAlGaAs-pGaAs-PAlGaAs double heterostructure diode.

Built-in Voltage in Heterojunctions 4 Fig. 33. Energy band diagram line up before equilibrium.

Built-in Voltage in Heterojunctions 5

Built-in Voltage in Heterojunctions Cont. 6

Built-in Voltage Method II: Gauss' Law 7 (145)

Forward-Biased NAlGaAs-pGaAs Heterojunction 8 Fig.34 Carrier concentrations in an n-p heterojunction (73) (161) Hole diffusion from pGaAs to the N-AlGaAs, (164) Electron diffusion from N-AlGaAs to the p-GaAs side, (162)

I-V Equation 9 Next we substitute the values of n po and p No in Eq. 107 (170)

I-V Equation and Current Density Plot 10 Here, we have used the energy gap difference  E g =E g2 -E g1. From Eq. 174 we can see that the second term, representing hole current density J p which is injected from p-GaAs side into N- AlGaAs, and it is quite small as it has [exp-(  E g /kT)] term. As a result, J ~ J n (x p ), and it is Fig. 38B Current density plots.

Carrier Confinement L4-1 new material Fig.41. Minority carrier concentrations in p-GaAs and in p-AlGaAs. Thus, the addition of P-AlGaAs at x=x p +d forces the injected electron concentration quite small. That is, it forces all injected carrier to recombine in the active layer. This is known as carrier confinement.

Energy band diagram of a double heterojunction 12 Fig. 42 Energy band diagram of a NAlGaAs-pGaAs-PAlGaAs double heterostructure diode.

Double heterojunction with a quantum well By reducing the thickness of p-GaAs layer to Å, we obtain a quantum well double heterostructure as shown schematically in Fig. 42B, page Al x Ga 1-x As GaAs ∆E V ∆E C 0 V(z) z -E G ∆E V -E G +∆E V z ∆E c = 0.6∆E g ∆E v = 0.4∆E g Fig. 42 B GaAs quantum well with finite barriers produced by AlGaAs layers.

14 Photon confinement in a waveguide region formed by double heterojunction layers (p.170) When electron and holes recombine in the GaAs layer, they produce photons. AlGaAs layers have lower index of refraction than GaAs layer. As a result it forms a natural waveguide. In the laser design example, we have mentioned various methods for the calculation of modes in such a slab waveguide. Also we need to calculate the confinement factor  of the mode. Confinement factor also determines the J TH. Generally, the confinement factor becomes smaller as the thickness of the active layer becomes narrower. This also depends on the index of refraction difference between the active and the cladding layers.

Fabrication of Diodes Interfacing of an n- and a p-type semiconductor forms a p-n junction (or diode). Experimentally, this is done by one of the following methods including: diffusion of p- impurities in n-Si, ion implantation of donor atoms in p-Si (generally this is followed by annealing to eliminate the damage to the lattice caused by high energy implantation), and epitaxial growth (depositing a p- layer on n-type substrate). In the case of diffusion or ion implantation, the impurity or dopant concentration is higher in the top layer than the substrate.

Diffusion from an infinite source: Predeposition p Fig. 23. The impurities distribution during predeposition. Note the increasing junction depth as a function of predeposition duration.

Junction depth measurements 17 Figure 24. (a) Sample before Diffusion (Width polished side up) (b) Sample after Diffusion (p-type) (c) Sample after back etch of p-Si diffused Layer. Figure 25. Dicing and Mesa Formation

Electrical Characterization of p-n diodes 18 Figure 26. (a) Left: Circuit connections for current source and voltage meter. (b) Right: Sample after Diffusion (p-type) Figure 27. C-V measurements Figure 29. Solar cell measurements