DE102005006026B4 - photodetector - Google Patents

Abstract

photodetector of a semiconductor material having at least two contacts, thereby characterized in that the semiconductor material is doped and a has such a small thickness that on the surface of the semiconductor material a depletion boundary layer is formed such that the interior of the semiconductor material has no mobile charge carriers.

Description

The The invention relates to a photodetector.

Out Photoresistors and photoconductors are known in the art Basis of nanopillars or Nanowhiskern known from semiconducting material. The nanopillars will be by self-organized epitaxial growth of the corresponding Semiconductor produced.

adversely follows in photoresistors as per stand the technology of the photocurrent of a change of lighting only delayed especially with a reduction in the illuminance. in the English usage, this property is also considered persistent referred to photoconductivity.

task The invention is to respond quickly to changes in lighting To provide a photodetector on a semiconductor basis.

The The object is achieved by a photodetector according to claim 1 and a method for the preparation of such solved according to the independent claim.

advantageous Embodiments result from the claims referring back.

Of the Photodetector according to the invention of a doped semiconductor material has at least two contacts on.

The Semiconductor material according to the invention has such a small thickness, that on the surface of the semiconductor material, a depletion boundary layer is so forms that the interior of the semiconductor material is not moving charge carrier having.

The thickness at which this condition is just reached is referred d _crit to below as the critical thickness.

in the Within the scope of the invention, it was recognized that the recombination rate photogenerated electrons and holes in the semiconductor material from the electrical potential barrier Φ between the interior of the semiconductor material and its surface dependent is. The lower the potential barrier, the higher the recombination rate. It was surprisingly found that the potential barrier is smaller, the smaller the thickness of the Semiconductor material is. If the critical thickness is undershot, the recombination speed increases quickly and the photodetector reacts faster.

in the Effect is the potential barrier between the interior of the photodetector and its surface smaller, as with a photodetector, which still contains charge carriers inside.

Φ_B

Schottkybarriere an der Oberfläche,

E_F

Ferminiveau und

E_L

Energie der Leitungsbandkante im neutralen Gebiet mit Ladungsträgern.

The general and independent of the form of the photodetector applies: Φ <Φ B - (E. L - E F ) With

Φ _B

Schottky barrier on the surface,

E _F

Fermi level and

E _L

Energy of the conduction band edge in the neutral area with charge carriers.

Advantageous has a photodetector according to the invention a very small dark current, because without lighting, so to speak no moving charge carriers for the Electricity transport available stand. By this is meant that the charge carrier density by the structure according to the invention the structure crucial, that is reduced by 10 powers over the prior art becomes.

There the speed of the photodetector at the recombination rate increases, however, the sensitivity may drop, depending on the desired Application one possible good compromise between speed and sensitivity with Help the diameter be adjusted.

By the term semiconductor material are particularly designed as nanocolumns and layers Semiconductor includes, although the invention is not limited to these forms.

If a column is so thick that there is still a neutral, that is not depleted, area without electric field inside the column, the potential barrier Φ is through the pebble barrier Φ _{B of} the pinned surface and the conduction band edge E _L in the non-depleted semiconductor material determined to Fermi level E _F.

wobei e die Elementarladung, e die Dielektrizitätskonstante des Halbleiters und Φ die Potentialbarriere einer dicken Säule mit neutralem Gebiet ist.The diameter of a column at which the neutral region just disappears can be calculated from the doping N _{D of} the semiconductor. In the case of photodetectors in the form of a nanopillar according to the invention, this is the critical thickness d _crit

where e is the elementary charge, e is the dielectric constant of the semiconductor, and Φ is the potential barrier of a thick neutral-area column.

For pillars with a smaller diameter than d _crit , the entire semiconductor material inside is charge carrier free. The electric field of the depletion boundary layer can only build up a smaller potential barrier between the interior of the column and the surface. The recombination rate increases accordingly.

The However, invention is by no means restricted to nano-columns. Rather, you can too thin semiconductor layers as photodetectors be used, wherein the layer thickness is chosen so that at the surface of Layer, a depletion boundary layer is formed such that the interior of which has no mobile charge carriers.

wobei e die Elementarladung, ϵ die Dielektrizitätskonstante des Halbleiters und Φ die Potentialbarriere einer dicken Schicht mit neutralem Gebiet ist.The relationship for the critical layer thickness, if only one surface creates a depletion boundary layer, is as follows:

where e is the elementary charge, ε the dielectric constant of the semiconductor and Φ the potential barrier of a thick layer with a neutral area.

If two opposite each other lying surfaces Create depletion margins, doubles the critical Thickness.

One Method for producing a photodetector therefore provides to arrange a semiconductor material on a substrate and to dope. It is characterized in that the thickness of the semiconductor material is so chosen low that will be on the surface of the semiconductor material, a depletion boundary layer is so forms that the interior of the semiconductor material has no mobile charge carriers.

The Semiconductor material may be selected in the form of a nanocolumn or a layer.

The thickness of the semiconductor material is less than the critical thickness d _crit according to formula 1 or formula chosen. 2

in the Further, the invention with reference to an embodiment and the accompanying figures described in more detail.

1 shows a scanning electron micrograph of a GaN nanocolumn 1 on an insulating substrate 2 with two electrical contacts 3a . 3b , From the prior art these photodetectors are known with column diameters larger than the critical thickness d _crit . They are characterized by a delayed recombination of the charge carriers.

2 schematically shows the relationship between the hatched depletion boundary layer 1 and the conduction (E _L ) and valence band profile (E _V ) of a nanocolumn depending on the diameter of the column. The example shows an n-doped semiconductor.

In the example, a column according to the prior art is shown on the right and the photodetector according to the invention on the left. reference numeral 2 refers to the neutral area inside the column.

For semiconductor materials with a depletion surface layer on the surface, as shown to the right in FIG 2 As shown by Fermilevel pinning, the photogenerated electrons and holes are separated by the electric field of this depletion layer. For example, with n-doped material, the electrons are collected inside the column as the holes move to the surface. The reverse is true with p-doped material. This spatial separation adversely delays the recombination of the generated charge carriers.

In the middle part of the 2 a column is shown in which the depletion layer at the surface just just penetrates into the entire column. The diameter of this column is referred to as the critical thickness d _krit .

A photodetector from a column with the diameter d _crit shows no increase in speed compared to a thicker column, but the dark current is advantageously much lower, because virtually no mobile charge carriers in the dark are available for current transport through the depletion boundary layer.

In the left part of the 2 is a column with a smaller thickness than d _crit shown. The depletion layer on the surface also penetrates into the entire column here.

However, since a smaller thickness according to the invention is chosen according to formula 1, the depletion edge layer can only have a lower potential barrier than in the middle and right part of FIG 2 build up.

The Recombination rate of photogenerated charge carriers increases, and a photodetector with this column becomes faster.

Just This effect is exemplary for a photodetector of a GaN nanopillar shown. The nanopillar was generated by MBE on silicon (111).

With a typical n-doping of 10 ¹⁷ cm ^-3 , a surface pinning Φ _B of 0.55 eV and a distance of the conduction band edge to the Fermi level E _F of 81.5 meV, the critical thickness d _{crit is} 200 nanometers. A photodetector from a nano-column with a diameter of 50 nm and two ohmic contacts ( 1 ) is then several orders of magnitude faster than a conventional device based on a column thicker than 200 nm.

On Reason of the above relationships and explanations can be for different Semiconductor materials and given doping can easily calculate the thicknesses below which a device such as the photodetector mentioned here particularly fast responding. It is conceivable that this procedure also applies to other components as transferred to photodetectors

Claims (11)

A photodetector of a semiconductor material having at least two contacts, characterized in that the semiconductor material is doped and has a small thickness such that on the surface of the semiconductor material, a depletion edge layer is formed such that the interior of the semiconductor material no having movable charge carriers. Photodetector according to the preceding claim, characterized by a thickness of the semiconductor material equal to or less than the critical thickness d _crit according to the following formulas 1 or 2:

with e = elementary charge, ε = dielectric constant of the semiconductor, Φ = potential barrier of a thick column with a neutral region and N _D = doping of the semiconductor. Photodetector according to the preceding claim, characterized by at least two times smaller thickness, in particular four times smaller thickness than the critical thickness d _crit Photodetector according to one of the preceding claims by GaN, AlN, InN, Si, Ge, GaAs, AlAs, InP, InAs or an alloy from one or more of these materials as a semiconductor material. Photodetector according to one of the preceding claims, characterized characterized in that this formed as a nanocolumn or nanolayer is. Photodetector according to the preceding claim, characterized through a nanopillar with a diameter of less than 90 nanometers in diameter. A photodetector according to any preceding claim, wherein which the contacts are designed as ohmic contacts. A photodetector according to any one of claims 1-6, wherein which at least one contact is designed as a Schottky contact. A method for producing a photodetector according to one of the preceding claims, wherein a semiconductor material disposed on a substrate and is doped, characterized in that in dependence of the doping the thickness of the semiconductor material is chosen so small that on the surface of the Semiconductor material such that a depletion boundary layer is formed, the interior of the semiconductor material has no mobile charge carriers. Method according to the preceding claim, characterized characterized in that a semiconductor material in the form of a nanopillar or a layer chosen becomes. Method according to one of the preceding claims 9 or 10, wherein a thickness of the semiconductor material is chosen equal to or less than the critical thickness d _crit according to the following formulas 1 or 2:

with e = elementary charge, ε = dielectric constant of the semiconductor, Φ = potential barrier of a thick column with a neutral region and N _D = doping of the semiconductor.